Hi there in Bolivia!
Etihad Airways conducts assessment days only by invitations!! In your case just send in your online application form and contact them by phone that you are willing to travel if they are looking for cabin crew in the U.S
Sometimes they also have Open days where you don`t need an invitation but it is only either in Beirut (Lebanon) or in Australia!
As Ashlea said before keep on checking their cabin crew assessment shedule! I think that they are full at the moment as there is no new schedule on their career website!
But just keep on!!!!!
Keep my fingers crossed for you and good luck!